Eccritotarsus catarinensis (Carvalho)(Miridae), a biological control agent for water hyacinth, Eichhornia crassipes (Mart.) Solms (Pontederiaceae) has failed to establish in some parts of South Africa due to climatic incompatibility. Some insects have the ability to change their thermal tolerance according to the temperatures they are exposed to, without a change in their genotype. This is known as phenotypic plasticity. Thermal tolerance of two populations of E. catarinensis from different climatic regions of the native range were tested using degree day models. This showed that each population’s thermal physiology matched their particular climate. However, after years of being reared under the same conditions in quarantine, experiments showed that their thermal physiologies have converged, which may be the result of adaptation or thermal plasticity. Samples of E. catarinensis have been collected from the hottest and coldest establishment sites in South Africa. Thermal plasticity will be investigated by determining their critical and lethal thermal limits. This will be done in summer and winter for comparison. Thermal limits will be tested before and after cold hardening using E. catarinensis from the BCRG mass rearing facility. Preliminary results have shown that insects from the colder site have lower thermal tolerances compared to those of the warmer site. If the thermal physiology of E. catarinensis has the ability to change due to phenotypic plasticity then it would be beneficial to exploit that characteristic, thus increasing their chances of establishment in colder regions. This will be done through cold hardening during mass rearing.